Fuel cells are electrochemical devices that convert chemical energy of a fuel directly into electrical energy without combustion. Accordingly, they have the potential to offer an environmentally friendly, light-weight, efficient, and renewable power source that make them particularly attractive for use in a wide variety of applications including electric vehicles and portable electronic devices.
Fuel cell operation is generally well understood. The typical fuel cell operates by extending an electrolyte between two electrodes: one operates as a cathode and the other an anode. Oxygen passes over one electrode and hydrogen over the other, generating electricity, water and heat. In particular, the chemical fuel contains hydrogen, which is fed into the anode of the fuel cell. Oxygen (or air) enters the fuel cell through the cathode. Encouraged by a catalyst, the hydrogen atom splits into a proton and an electron, which take different paths to the cathode. The proton passes through the electrolyte, while the electrons create a separate current that can be used before they return to the cathode where they are reunited with the hydrogen and oxygen to form a molecule of water.
There are a wide variety of fuel cells either currently available or under development that use a variety of different fuels and electrolytes. These different types of fuel cells include phosphoric acid, molten carbonate, solid oxide, alkaline, direct methanol, and regenerative fuel cells.
Among these available fuel cells is a type commonly referred to as a Proton Exchange Membrane (“PEM”) fuel cell. PEM fuel cells operate at relatively low temperatures, have the potential for high power density, and can vary their output quickly to meet shifts in power demand. According to the United States Department of Energy, “they are the primary candidates for light-duty vehicles, for buildings, and potentially for much smaller applications such as replacements for rechargeable batteries.”
The PEM of a PEM fuel cell typically includes porous carbon electrodes, which are platinum loaded to provide a catalyst region which then sandwiches a proton conducting electrolyte material therebetween. The proton conducting electrolyte material is typically a perfluorinated sulfonic acid polymer. Once sandwiched between the electrodes, the membrane-electrode assembly is then heated to cause the electrolyte to impregnate the platinum loaded electrode, as well as form a bond between the layers. The membrane-electrode assembly in then placed in a manifold structure in order to deliver fuel and oxidant to the electrodes.
In order for fuel cells, including PEM fuel cells, to be used in portable electronic devices and as replacements to conventional batteries and the like, they must be compact and light weight, but still provide enough electrical current to power the devices to which they will be used. However, the amount of current generated by these types of fuel cells is proportional to the volume of available fuel and the amount of PEM surface area available to interact with the fuel. Accordingly, in order to make these fuel cells small enough for such applications, the manifold structure must be suitably compact, while the volume of available fuel and related PEM surface area remains large enough to actually produce enough electricity to power the devices to which the fuel cell will be used.
As shown in prior art FIGS. 1 and 2, scientists have had some success in producing miniaturized PEM fuel cells 10 using a conventional silicon wafer 12 as the manifold structure 14. In general, these scientists used conventional thin-film deposition, patterning, and etching processes to define a plurality of cup-shaped chambers 16 within the silicon wafer 12. Each cup-shaped chamber 12 is then filled with a suitable fuel, and the previously described membrane-electrode assembly 18, which preferably has an anode 20, cathode 22, and proton exchange membrane 24, is operably secured to the manifold structure 14 thereby defining an oxygen (or air) region 26 and fuel region 28 divided by the membrane-electrode assembly 18.
These scientists were able to produce electricity from their miniaturized structure. However, its design unnecessarily limits the amount of electricity it can produce and needlessly complicates its manufacture, thereby limiting its usefulness for commercial purposes. For example, the amount of PEM 24 surface area available to interact with the fuel is limited. Moreover, it is difficult and time consuming to initially manufacture and then consistently fill a plurality of cup-shaped chambers 16 with fuel. It is also difficult to ensure uniform flow through all of the available cup-shaped chambers 16 during operation of the fuel cell. Accordingly, known methods for overcoming such difficulties necessarily increase the cost of producing each fuel cell or alternatively limit of the amount of electricity they produce.